U.S. Pat. No. 5,467,636 describes a sensor in which a planar sensing element is secured in gas-tight fashion in a ceramic shaped element by way of a sensing element seal. The sensing element seal is a glass seal which is provided, in the form of a fusible glass seal, in a depression which surrounds the sensing element and is introduced on the exhaust gas-side ceramic shaped element, and thereby separates a reference gas space from a measured gas space.
Another sensor is described in U.S. Pat. No. 4,596,132. As described therein a sensing element is mounted directly in a reference gas space-side housing part of a metal housing by way of a sensing element seal. The sensing element seal is formed by a fusible glass seal which encloses the reference gas space-side end of the sensing element together with the contacted connector cables. The sensing element operates without a reference atmosphere.
A sensor according to the present invention achieves a secure, gas-tight, and gasoline-resistant sealing of the sensing element. The sensor is of simple construction in terms of assembly engineering, and to that extent is economical to manufacture. The installation space available on the reference gas side is used in order to arrange the sensing element seal as far away as possible from the hot exhaust gas. As a result, the various coefficients of thermal expansion of the sensing element seal and solid electrolyte material of the sensing element, and the reaction behavior of the material of the sensing element seal with the solid electrolyte material of the sensing element, have less of an effect, thus creating a sensing element seal that is crack-free and reliable, at high temperatures and in the presence of temperature cycling, over its entire utilization life.
A particularly gas-tight and gasoline-resistant sensing element seal is achieved by way of a glass seal, the glass seal being introduced into the receptacle in the form of fusible glass. A further limitation in the heat flux toward the glass seal is achieved by a thermal insulation element that is arranged between the ceramic shaped element and fusible glass seal and is made of a material with poor thermal conductivity. It has proven advantageous in this context to use a presintered steatite ring which is deformed by the application of pressure into a powder packing. One advantageous embodiment, which allows the use of a preassembled subassembly made up of ceramic formed element, sensing element, inner metal sleeve, and glass seal, moreover consists in the fact that a further presintered steatite ring, which secures the ceramic shaped element in the housing by sealing, is inserted between the ceramic shaped element and the housing.
A further advantageous embodiment with a preassembled subassembly is made possible by pre-securing the sensing element in the housing. In this context, the sensing element is secured, prior to manufacture of the sensing element seal, by way of a powder packing configured between two ceramic shaped elements. The powder seal acts simultaneously as an insulator with respect to the thermal conduction occurring during manufacture of the fusible glass seal, and as an additional primary seal. The seal arrangement made up of the sensing element seal and powder packing thus forms a dual-action seal which has an additionally favorable effect on continuous operation of the sensor. The advantage of this embodiment moreover lies in the fact that no assembly forces act on the sensing element seal which is subsequently manufactured.
A further reduction in the influence of the coefficients of thermal expansion of the solid electrolyte material of the sensing element and the fusible glass is possible by the fact that a ceramic shaped element, which surrounds the sensing element and has a coefficient of thermal expansion very closely matched to the solid electrolyte material of the sensing element, is inserted into the fusible glass.